Geckos’ tiny toe hair may solve big problems

by Aditi Mishra – Geckos are capable of cool feats – like growing their tails back and defying gravity. For centuries they have kept us wondering. So how do geckos defy gravity? Everything from sticky secretions to tiny hooks and suction cups have been explored. As it turns out, nature outsmarts us again – the answer is unexpectedly simple.

Suction cups and hooks were ruled out because none of them were observed in geckos. Since dead geckos are still sticky, it was implied that the famous gecko grip couldn’t be a function of secreted glue. Only 3 possibilities remained: surface tension, static electricity and Van der Waal forces. Using surface tension would mean that the gecko sticks to walls like wet paper sticks to glass. For static electricity to be generated, charge must be exchanged between two surfaces: one side becomes positive and the other, negative. The opposite charges attract and makes the surfaces stick to each other. It is the same force that makes thermocol stick to glass when rubbed together. To understand the third possibility, let’s understand Van der Waal forces better. Van der Waal forces are present everywhere – it is the force that binds us, surrounds us and keeps us together. Every molecule has a tiny cloud of charge around it which fluctuates with time. These fluctuations in charge distribution create a temporary positive and negative pole. The attraction between the positive and negative poles gives rise to Van der Waal forces.

Scientists have meticulously tested all the possibilities. They measured if geckos could scale hydrophobic surfaces – yes, they could. German scientist, W.D. Dellit blew ionized air on the feet of geckos to see if geckos would slip -no, they didn’t. Geckos’ strong grips on water-repellent surfaces imply that surface tension isn’t the key. If geckos use static electricity, ionized air should neutralize the charges and send them sliding off.

So Van der Waal forces were the last contenders standing– even though it is a very small force per unit area. As Sir Arthur Conan Doyle eloquently put it –

Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.

Gecko foot hairs. Coloured scanning electron micrograph (SEM) of hairs on the underside of a gecko’s foot (family Gekkonidae). These microscopic hairs enable the gecko to cling to very smooth surfaces such as windows and ceilings. Geckos are slender nocturnal lizards. They feed on insects and are found in a wide range of habitats, from deserts to rainforests. Magnification: x595 at 6x7cm size.

To solve this enigma, scientists examined geckos’ feet closely. Geckos have a unique condition – each of their five toes are covered with at least 5 million spatula-like structure called “setae”. Additionally, geckos have a peculiar habit of curling and uncurling their toes rapidly. Scientists observed that a gecko spreads its toe hair and setae every time it puts its foot down. The 5 micron long setae (which can be compared to split ends on your toe hair) come as close as 3Å to the surface. Subtle intermolecular interactions like Van der Waal forces are possible at such short ranges. The strength of these interactions depend on the geometry of attachment sites alone. The material of the setae and surface has nothing to do with it.

The behavior of geckos lends more support to the theory. Geckos regularly curl and uncurl their toes and this changes the geometry of the setae. Changing geometry of setae changes the strength of the acting Van der Waal forces. Hence, attaching and detaching from the surface may happen by a simple change in posture.

To test the causation, scientists tested synthetic foot-hair tips of different sizes (in nanometer range) from two materials. The force of attachment generated was dependent on geometry of the surface rather than the material. This hinted that Van der Waal forces could indeed be responsible for the grip. The force generated by one seta was small but all the setae on one foot exerted 10 N. Hence, a gecko can comfortably hang from the ceiling using one foot alone. All the setae used together can produce a whopping 280 N.

So this settled the debate – or so we thought!

In 2014, a group from the University of Waterloo explored how contact electrification could affect adhesion of geckos. Contact electrification is the phenomenon of surfaces exchanging charges and acquiring static electricity when rubbed together. The group studied geckos on two surfaces: Teflon AF and polydimethylsiloxane (PDMS). Since both of these are insulating, the geckos’ toes became positively charged while surfaces became negative. Geckos stick better to Teflon AF as compared to PDMS, and this is linked with higher contact electrification on the former. Thus, sometimes, surface properties do matter. Static electricity could be especially helpful on smooth surfaces.

This observation doesn’t necessarily negate Dellit’s observations. Ionized air ions (typically of 0.4 nm radius) are too big to neutralize the charges between setae and surface (contact distance between setae and surface is 0.3 nm).

Till date, it is unclear how useful static electricity would be to geckos in the real world. The bigger question of how geckos grip remains contested as well. Perhaps, geckos use both the strategies in the appropriate contexts. So, was all this vain? Not at all: enroute to the final answer, humans derived a lot of inspiration to solve some other problems. DARPA, an agency working for the US Department of Defense, has come up with “Geckskin”, a reversible adhesive surface which uses Van der Waal forces. A mere 4-square-inch of it is strong enough to support a grown man hanging from the ceiling. Scientists are exploring biodegradable tapes of a similar design to replace surgical sutures. Static electricity – another small force – is currently used to allow Robobees to perch.

Sometimes it’s the journey and not the destination. When it comes to understanding the gecko’s grip it certainly seems to be so.